In a magnetic bearing device, an object is supported electromagnetically in a contactless manner. One or more degrees of freedom of the object are controlled actively by providing position or displacement sensors, feeding the sensor signals to a controller, deriving control currents or control voltages based on the sensor signals, and applying these currents or voltages to electromagnetic actuators with the aid of power amplifiers. In this way, stable levitation of the object to be supported is achieved.
In an important example, a rotor is suspended in a magnetic bearing device for rotation around a rotor shaft. The long axis of the shaft is designated by z, and two mutually orthogonal directions perpendicular to the shaft axis are designated by x and y. Usually five degrees of freedom (three translational and two rotational degrees of freedom) are controlled. In principle, five sensors and the same number of actuators are sufficient for this purpose. Often, however, a higher number of actuators is employed, usually ten, organized in five pairs.
For control of radial motions (translational and tilting motions in the x and y directions), usually two radial bearing units in an upper and a lower position along the z axis are provided. Often, in each radial bearing unit, two pairs of actuators are present for controlling displacements of a shaft section in the ±x and ±y directions, respectively. Likewise, an axial or thrust bearing unit with one pair of actuators is usually present for controlling displacements in the ±z direction. A bias current may be provided to each actuator for setting the operating point.
Each actuator is usually connected to an individual power amplifier by two wires. The power amplifiers are usually housed in a distinct amplifier unit, which can be well removed from the actuators. For the above example of ten actuators, this results in a total number of twenty wires leading from the amplifier unit to the actuators over a significant distance. This high number of wires makes cables and connectors expensive and may also affect reliability.
Different measures have been suggested in the art for reducing the number of wires.
On one approach, biasing of reluctance type actuators is carried out by permanent magnets instead of providing bias currents. The permanent magnets provide a bias magnetic field. (Ulbrich, H.; Wang, Y.-X.; Bormann, J.: Magnetic Actuator Design for Mechanical Engineering Applications. Proceedings of the 4th International Symposium on Magnetic Bearings, Zürich 1994, pp. 377-382.) Then each pair of actuators in each bearing unit may be connected in series with opposite polarity, in such a way that a current through the actuators causes an increase of the magnetic field in the first actuator and a decrease of the magnetic field in the second actuator in the pair. Thereby the number of required wires between the actuators and the amplifier unit may be reduced.
Another approach for reducing the number of wires is the use of a common return wire for several actuators, typically for the connection to ground. In a system with ten actuators, the number of wires may thus be reduced from twenty to eleven. However, in this approach the load requirements of the common return wire are much higher than for the other wires, and the overall power handling capabilities of the wiring may even be increased compared to traditional wiring.